1. Field of the Invention
This invention relates to microporous membranes, and more particularly to reinforced microporous membranes suitable for the filtration of aqueous fluids, such as biological liquids and high purity water used in the electronics industry.
2. Prior Art
Nylon microporus membranes are well known in the art. For example, U.S. Pat. No. 3,876,738 to Marinaccio et al (1975) describes a process for preparing nylon microporous membranes by quenching a solution of a film forming polymer in a non-solvent system for the polymer. European Patent Application No. 0 005 536 to Pall et al (1979) describes a similar type nylon membrane and process.
Other type polymeric microporous membranes, including nylon, and processes for producing such membranes are described, for example, in the following U.S. Pat. Nos.
3,642,668 to Bailey et al (1972); PA1 4,203,847 to Grandine, II (1980); PA1 4,203,848 to Grandine, II (1980); and PA1 4,247,498 to Castro (1980).
Commercially available nylon microporous membranes are available from Pall Corp., Glencove, N.Y., under the trademark ULTIPOR N66. Another commercially significant membrane made of polyvinylidene fluoride is available from Millipore Corp., Bedford, Mass., under the trademark DURAPORE. This membrane is probably produced by the aforementioned Grandine, II patents.
Additionally, the Assignee of this application is selling two types of cationically charge modified nylon microporous membranes under the trademark ZETAPOR. These membranes are described and claimed in U.S. Ser. No. 268,543 filed on May 29, 1981 to Barnes et al, now U.S. Pat. No. 4,473,475, and U.S. Ser. No. 314,307 filed on Oct. 23, 1981 to Ostreicher et al, now U.S. Pat. No. 4,473,474. Barnes et al describes the use of charge modified membranes for the filtration of high purity water (18 megohm-centimeter resistivity) used in the electronics industry; and Ostreicher et al describes the use of charge modified membranes for the filtration of parenteral or body liquids.
Generally, the methods for producing a microporous membrane include spreading a casting solution on a substrate to form a thin film thereon, which eventually is formed into a microporous membrane. For example, the aforementioned Marinaccio et al process produces a microporous membrane through the quench technique. This technique involves casting or extruding a solution of a film-forming polymer in a solvent system, casting the solution on a substrate to form a film and quenching the film in a bath which includes a non-solvent for the polymer. The Pall application involves a similar type process.
The aforementioned Pall application is the most relevant known prior art relating to the formation of microporous membranes into useful laminates for filtration discs, cartridges, etc. As described by Pall a polymer solution is cast on a substrate which may be porous or non-porous to form a thin microporous membrane on the substrate. The membrane produced may then be removed from the substrate, washed and combined, i.e. laminated, with another similar type membrane to form a dual layer membrane. This dual layer membrane is then dried under restraint forming a membrane having particle removal characteristics superior to those of the individual layers. Pall indicates that the membrane layers can have the same or different porosities, have tapered pores or uniform pores, and be supported or unsupported. If a membrane layer is supported the substrate upon which it is cast is not removed from the membrane, i.e. the substrate is an integral part of the membrane. Such a substrate is preferably a porous material that is wetted by the polymer solution so that the solution will penetrate the substrate during casting and become firmly attached thereto during formation of the microporous membrane. Pall states that such supported membranes have a somewhat higher pressure drop than unsupported membranes (i.e. lower flow rate for a given pressure differential), and that the increase in pressure drop is small if the supporting web has an open structure.
The problems associated with the dual layer type filtration membranes of Pall, are comparatively major when one considers the uses for such membranes (e.g. the filtration of parenteral or body liquids, high purity water, pharmaceuticals, beverages, etc.), the small pore size of the membranes (i.e., less than about 1.2 microns and usually about 0.2 microns), the manipulation required to form the membrane into usable forms, e.g. cartridges, and that preferably the membrane be useful with either side facing upstream. It has been found by the Assignee in the production of the commercial forms of the membranes described in the aforementioned Barnes et al and Ostreicher et al applications, that a non-reinforced dual layer membrane is generally too fragile, e.g. tends to crack when pleated to form filtration cartridges and generally causes too high a reject rate for cartridges used as sterilizing filtration media. In attempting to solve this problem by reinforcing the membranes by casting directly onto a reinforcing web and then laminating the layers together, it was found that very careful control over the casting conditions had to be maintained to avoid streaking, non-uniform wetting of the web, pin-holes, air-bubbles, etc. More specifically, it was discovered that air bubbles tended to form in the membrane in the areas of crossover between the pieces of yarn forming the web, and that such air bubbles cause objectionable voids or pockets in the final membrane. In some instances, such air bubbles resulted in an opening through the membrane which rendered it useless. Additionally, as indicated in Pall, the pressure drop across the membrane tended to be too high (or the flow too low).
Additional prior art of interest, none of which solve all of the above problems, are the following U.S. Nos.:
2,806,256 and 2,940,871 to Smith-Johannsen (1960) describes producing a microporous film by fusing fine particles of thermoplastic material while dispersed throughout a viscous or continuous inert matrix medium and then dissolving out the inert matrix medium. The membrane may be used for filtration. The thermoplastic material may be a polyamide. The dispersion may be calendered, knifed, molded, dip coated, or rolled onto a backing material to reinforce the film. The backing material is desirably at least as porous as the microporous film itself.
2,960,462 to Lee et al (1960) describes the use of laminated permeation membranes for the separation of organic chemical mixtures. The laminated membrane consists of a thin layer or film of a selective membrane material and a thicker film of a membrane material having a higher permeation rate. The laminated membrane is employed so that the film layer having the higher permeation rate is in contact with the mixture of the chemicals to be separated.
3,190,765 to Yuan (1965) describes, polymeric films adherently united to fibrous substrates. The primary use is as a substitute leather, although the materials may be used as filters. The products are produced by applying a layer of a polymer-containing solution to one or both sides of a flexible porous fibrous substrate. The leather when bathed and dried in the manner described therein, becomes a microporous polymeric layer integrally united to the fibrous substrate. The sheet material formed has a high permeability to vapors and relatively low permeability to liquids, i.e., it is breatheable yet water repellent.
3,100,721 and 3,208,875 to Holden (1965) describes the manufacture of leather like sheet materials having a microporous/durable elastomeric material in superposed adherence with a fabric or other porous fibrous sheet material. The uses for such vapor permeable sheets are as shoe uppers, upholstery and clothing. The sheet products are produced by applying a layer of a solution containing a polymer to one or both sides of a flexible porous fibrous substrate.
3,389,045 to Jones et al (1968) describes a method of producing thin thermoplastic films having a multicolored ombre. Such films are used for decorative curtains, wearing apparel, surface covering for floors, walls, furniture, etc. The process involves the controlled calendering of several separate thermoplastic materials of different colors.
3,551,244 to Forester et al (1970) describes an ultrathin polymer film on a support membrane for use as a reverse osmosis membrane.
3,556,305 to Shorr (1971) describes a reverse osmosis membrane comprising (a) an anisotropic membrane, (b) a very thin layer of a film forming adhesive polymer and (c) a very thin diffusive type membrane film overlying the adhesive polymer.
3,615,024 to Michaels (1971) describes an anisotropic polymeric membrane which is formed by casting the film on an impermeable surface, e.g., a glass or metal surface, or a permeable surface, e.g. paper. Particularly advantageous materials for use are permeable non-wettable, non-woven sheets, for example, polyolefin fiber felts.
3,679,540 to Zimmerman et al (1972) describes reinforced microporous films used for sterile packaging, hospital bed sheets, and pillow liners. The reinforced films are produced by laminating a specifically characterized microporous polymer film onto a specifically characterized microporous film. The reinforced film is said to have increased strength and high permeability.
3,709,841 to Quentin (1973) describes sulphonated polyarylether-sulphone ion exchange membrane combined with a reinforcing support.
3,721,596 to Drake (1973) describes an osmotic desalination membrane produced by immersing an apertured support in a casting solution and then allowing the film to set.
3,744,642 to Scala et al (1973) describes a desalination membrane having a membrane layer of polymeric material, e.g. polyamide, in contact with a substrate. The membranes produced on the substrate are generally homogenous and between 0.1 and 1.0 mils thick. The membrane can coat one or both sides of the substrate and can be applied continuously. The substrate can be interwoven or felted fibers of paper, plastic, glass, etc. The thin polymeric film which is formed comprises interlocked polymer chains in contact with each other and which generally extend into the pores within the substrate.
3,762,566 to Del Pice (1973) describes a supported semi-permeable membrane produced by impregnating the surface of a porous support with a non-solvent for the film forming polymer used in a subsequently applied casting solution.
3,912,834 to Imai et al (1975) describes a reinforced ultrafiltration or reverse osmosis membrane. Porous backing materials are impregnated prior to coating with a film-forming polymer solution, with a liquid in which the polymer is insoluble to the extent that at least one of the surfaces of the backing material has remained substantially free from the liquid while other portions, especially the interior thereof become wet. The coating solution of the film-forming polymer in a volatile solvent is then applied to the surface of the backing material and the coating layer is then gelled or otherwise treated to give a selectively permeable membrane on the backing material.
3,951,815 to Wrasidlo (1976) describes a composite semi-permeable membrane wherein an ultra thin film is formed in situ upon a microporous substrate. More specifically, a polysulfone membrane is cast on to a tightly woven dacron cloth. This fabric reinforced polysulfone substrate is then presoaked in a cyanoethylated polyethylenimine aqueous solution. The membrane is dried and disposed upon a glass plate and immersed again in the same aqueous solution to produce the composite "polyamide membrane."
4,005,012 to Wrasidlo (1977) describes a process for producing a semi-permeable anisotropic membrane useful in reverse osmosis processes. The membranes are prepared by forming a polymeric thin film possessing semi-permeable properties, on a microporous support.
4,026,977 to Bourganel (1977) describes a process for producing an anisotropic membrane by casting a solution of a sulfonated polyaryl ether/sulfone on a support immersing the support coated with the layer of polymer in a coagulating bath and then recovering the resulting membrane. The support can also be covered with a reinforcement material intended to reinforce the membrane. This reinforcement material can consist of a woven fabric, a net or knitted fabric.
4,061,821 to Hayano et al (1977) describes a semi-permeable composite membrane consisting of a porous substance and a reinforcing material embedded therein. The method of producing the membrane comprises impregnating the reinforcing material with a coagulating liquid. The reinforcing cloth may be polyester. The semipermeable membrane may be copolymers of acrylonitrile, cellulose acetate, polyamides, polyurethanes, polysulfones.
4,201,838 to Goldberg (1980) describes a laminated microporous article consisting of at least two layers or plys of thermoset or thermoplastic microporous material and a polyester non-woven, heat bonded web. The laminate is particularly useful as a battery separator.
4,207,182 to Marze describes a screen supported or reinforced semi-permeable membrane which is embedded in a flexible support of woven fabric, non-woven fabric or a net. The membrane is produced by casting a solution of a polymer on the reinforcement material followed by removal of the solvent, e.g. by evaporation and/or coagulation.
4,214,994 to Kitano (1980) describes a reverse osmosis membrane for use to purify sea water into plain water. The membrane comprises a skin layer as an active layer and a gel layer to support the skin layer. An improvement of flux, i.e., passing rate of purified water, is achieved by burying a porous sheet, such as plain woven cloth or non-woven fabric, in the gel layer.
4,244,817 to Yaginuma (1981) describes a process for preparing a semi-permeable membrane comprising coating a thin polyamine film on a reinforced microporous substrate and then subsequently treating to crosslink the polyamine.
4,277,344 to Cadotte (1981) describes a process for making a reverse osmosis membrane by coating a porous support layer with a polyamine component and then contacting the thus coated support with the polyacyl halide component to initiate polymerization.